1. Field of the Invention
This invention relates to semiconducting ceramics of barium titanate (BaTiO.sub.3) systems.
2. Description of the Prior Art
It is well-known in the art that semiconducting ceramics of barium titanate systems are produced by adding a barium titanate composition (BaTiO.sub.3) with at least one of rare earth elements or antimony (Sb), bismuth (Bi), niobium (Nb) and tantalum (Ta) as an activator for making the composition semiconductive and that the semiconducting ceramics of the barium titanate sintered family thus produced are employed as various electronic elements such, for example, as positive temperature coefficient characteristic of resistivity (PTCR) thermistors and the like, for utilization of their specific resistance-temperature coefficient characteristic of resistivity. In this case, it is often required that a change in the specific resistance with temperature represents a high gradient of a temperature at which the positive temperature coefficient characteristic of resistivity is exhibited and that the specific resistance at room temperature is controlled at a desired value. To comply with the requirements, it is the practice in the art to selectively alter firing conditions such as firing temperature, firing time, firing atmosphere and so on in the ceramics manufacturing process or add a small amount of iron (Fe), manganese (Mn) or other element to the semiconducting ceramics composition, thereby providing ceramics having specific resistance value at room temperature which is controllable and whose specific resistance undergoes a rapid and large change over the range of the temperature at which the positive temperature coefficient characteristic of resistivity is exhibited.
However, changing the firing conditions is undesirable since it is difficult to simultaneously control the specific resistance value and the range of its change with temperature as desired. In addition, it would require complex operations and controls to perform the process of mass production of the semiconducting ceramics. Further, the addition of a small amount of iron (Fe), manganese (Mn) or like element involves many difficulties in the manufacture of the semiconducting ceramics for the following reasons. Namely, a permissible range for the amount of such element added, for example, iron, is as narrow as 0.01 percent by weight relative to barium titanate, and the range for manganese is also 0.05 percent by weight at maximum. In addition, the specific resistance of the semiconducting ceramics is extremely susceptible to the influence of Fe or Mn, so that the amount of these elements added must be strictly controlled to obtain a desired specific resistance value and a desired change in the specific resistance with temperature.